The recombinant translationally controlled tumor protein (rTCTP) was initially reported as a tumor-associated protein in mouse tumor and mouse erythroleukemic cells, as well as human tumor cells. It is a growth related protein and its homologues from mammalian species have been shown to have extensive similarities about 95% in rat (p21), mouse (p21) and human (p23). In contrast to the earlier report that TCTP is a tumor-specific protein, recent studies indicated that this protein is ubiquitously expressed in many different tissues. TCTP expression has been reported in many cell types including erythrocytes, hepatocytes, macrophages, platelets, keratinocytes, erythroleukemia cells, gliomas, melanomas, hepatoblastomas and lymphomas. However, TCTP expression in brain or neuronal cell systems has not been known. Recently, decreased level of TCTP expression in brain was observed in patients with Alzheimer's disease and Down's syndrome. This may suggest a significant role of TCTP in neurological systems.
In this study, the functional role of rTCTPs in dopamine release from neuronal like PC12 cells was investigated. The effect of rTCTP on dopamine release was examined focusing on the relationship between role of Ca^(2+) and neurotransmitter release. The rTCTPs evoked dopamine release in a concentration- and a time-dependent manner. Interestingly, the rTCTP-driven effect on dopamine release was not dependent on extracellular Ca^(2+), in contrast with the increase in [Ca^(2+)]i. Further investigating of intracellular Ca^(2+) dependency of rTCTP-induced dopamine release, we used BAPTA-AM, a membrane-permeant chelator of cytosolic Ca^(2+). The treatment of BAPTA-AM did not produce a significant inhibition of rTCTP-evoked dopamine release. Using nifedipine, a voltage-gated L-type Ca^(2+)-channel blocker, was tested for its effect on TCTP-induced dopamine release and Ca^(2+)-current. Further evidence was shown that rTCTP can induce dopamine release in a Ca^(2+)-independent pathway.
To further investigate the mechanism involved in rTCTP-induced dopamine release in PC12 cells, the involvement of phospholipase A_(2) (PLA_(2)) in this process was examined using inhibitors of cPLA_(2), sPLA_(2), or iPLA_(2). rTCTP-stimulated dopamine release was partially inhibited by a Ca^(2+)-dependent cPLA_(2) inhibitor (AACOCF_(3)) and a Ca^(2+)'-dependent sPLA_(2) inhibitor (TEA-PC). On the other hand, significant inhibitory effect on rTCTP-induced dopamine release was shown with a selective inhibitor of Ca^(2+)-independent iPLA_(2) (BEL).
These results may suggest that rTCTP can induce dopamine release from neuronal like PC12 cells via a Ca^(2+)-independent pathway in a dose-and time-dependent manner and PLA_(2) may play a role in the process.